Helium-3 from Tritium Decay

Nearly all terrestrial helium-3 is a decay product of tritium, formed as tritium undergoes beta decay with a 12.3-year half-life.

The decay that makes helium-3

Tritium, hydrogen-3, is a radioactive isotope with one proton and two neutrons. It undergoes beta-minus decay: one neutron converts to a proton, emitting an electron and an antineutrino. The product is helium-3, which has two protons and one neutron. The decay releases a low-energy electron, with a maximum energy of about 18.6 kiloelectronvolts, and is otherwise clean, leaving stable helium-3 behind.

Half-life and accumulation

Tritium has a half-life of about 12.3 years. This means a stock of tritium converts to helium-3 at a steady, predictable rate: roughly 5.5 percent of the remaining tritium turns into helium-3 each year. Helium-3 therefore accumulates in any container of stored tritium and can be periodically harvested by pumping off the helium gas.

Where the tritium comes from

Tritium is scarce and is produced deliberately, historically in reactors and, in a fusion context, by neutron capture on lithium in a breeding blanket. Because helium-3 is a byproduct of stored tritium, the helium-3 supply is downstream of whatever tritium happens to exist and decay. This is the root reason the isotope is scarce and its supply inflexible, as discussed under supply and demand.

A worked estimate

The fraction of tritium decayed after time t is one minus two to the power of minus t over the half-life. Over one year that is one minus two to the power of minus 1 over 12.3, which is about 0.055, consistent with the 5.5 percent per year figure. This simple relation lets operators forecast how much helium-3 a given tritium inventory will yield over time.

import math
half_life_yr = 12.3
def he3_fraction(years):
    return 1 - 2 ** (-years / half_life_yr)
for y in (1, 5, 12.3):
    print(y, round(he3_fraction(y), 3))
# 1 -> 0.055, 5 -> 0.245, 12.3 -> 0.5

• Tritium beta-decays to helium-3 with 12.3-year half-life
• About 5.5 percent of a tritium stock becomes helium-3 per year
• Helium-3 is harvested by pumping gas off stored tritium
• Supply is downstream of how much tritium exists